Composite anode structure for aqueous electrolyte energy storage and device containing same

ABSTRACT

An anode electrode for an energy storage device includes both an ion intercalation material and a pseudocapacitive material. The ion intercalation material may be a NASICON material, such as NaTi 2 (PO 4 ) 3  and the pseudocapacitive material may be an activated carbon material. The energy storage device also includes a cathode, an electrolyte and a separator.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/972,409, filed Aug. 21, 2013, which claims the benefit of U.S.Provisional Patent Application No. 61/736,137, filed Dec. 12, 2012, bothof which applications are entitled “Composite Anode Structure forAqueous Electrolyte Energy Storage and Device Containing Same”. Theentire contents of the foregoing applications are incorporated herein byreference.

FIELD

The present invention is directed to ensembles of electrochemical cellsand in particular to hybrid energy storage devices.

BACKGROUND

Small renewable energy harvesting and power generation technologies(such as solar arrays, wind turbines, micro sterling engines, and solidoxide fuel cells) are proliferating, and there is a commensurate strongneed for intermediate size secondary (rechargeable) energy storagecapability. Energy storage batteries for these stationary applicationstypically store between 1 and 50 kWh of energy (depending on theapplication) and have historically been based on the lead-acid (Pb acid)chemistry. The batteries typically comprise a number of individual cellsconnected in series and parallel to obtain the desired system capacityand bus voltage.

For vehicular and stationary storage applications, it is not unusual tohave batteries with bus voltages in the hundreds or thousands of volts,depending on application. In these cases, where many units are connectedelectrically in series, there is typically an inherent need for thesecells to be as similar to each other as possible. In the event that thecells are not similar enough, a cell-level monitoring and controllingcircuit is commonly necessary. If some set of cells in a string of cellshave lower charge capacity than others in the same string, the lowercapacity cells will reach an overcharge/undercharge condition duringfull discharge or charge of the string. These lower capacity cells willbe de-stabilized (typically due to electrolyte corrosion reactions),resulting in diminished lifetime performance of the battery. This effectis common in many battery chemistries and is seen prominently in theLi-ion battery and in the supercapacitor pack. In these systems, costlyand intricate cell-level management systems are needed if the cells arenot produced to exacting (and expensive) precision.

SUMMARY

An embodiment relates to an anode electrode for an energy storage deviceincludes both an ion intercalation material and a pseudocapacitivematerial. The ion intercalation material may be a NASICON material, suchas NaTi₂(PO₄)₃ and the pseudocapacitive material may be an activatedcarbon material.

Another embodiment relates to a method of operating the energy storagedevice comprising a plurality of electrically connected electrochemicalenergy storage cells, wherein each cell comprises a negative anodeelectrode comprising both an ion intercalation material and anelectrochemical double layer capacitive and/or pseudocapacitivematerial, a positive cathode electrode, a separator, and an aqueouselectrolyte, the method comprising charging and discharging theplurality of electrochemical energy storage cells, wherein theelectrochemical double layer capacitive and/or pseudocapacitive materialprotects the ion intercalation material from corrosion by getteringhydrogen species that evolve during the charging step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an individual electrochemical cell according to anembodiment.

FIG. 2 is an exploded view of a device according to an embodimentcontaining four prismatic/parallel stacks connected in series inside awalled polymer housing.

FIG. 3 is a perspective view of the housing of FIG. 2.

FIG. 4 is a perspective view of a stack of seven housings shown in FIG.3 which are connected electrically in series. This stack of housingsincludes 28 prismatic/parallel stacks of cells connected in series toform an about 56 V battery system.

FIG. 5 is a schematic perspective view of multiple stacks of housingsconnected electrically in series.

FIG. 6A is plot of cell potential in volts versus capacity in arbitraryunits from a single cell with a λ-MnO₂ cathode structure and a compositeanode containing activated carbon and NaTi₂(PO₄)₃. The different voltagecharacteristics of the capacitive/pseudocapacitive, overcharge andintercalation operating modes are denoted in different regions as afunction of capacity by respective letters C, OC and I.

FIG. 6B is a plot showing the voltage vs. capacity (arbitrary units)profiles (for symmetric constant current charge/discharge studies) insingle cells containing anodes with different NaTi₂(PO₄)₃/activatedcarbon mass ratios of 20:80, 60:40 and 80:20.

FIG. 7A is plot of voltage vs. capacity collected from a cell with ananode containing a 1:1 mass ratio of activated carbon/NaTi₂(PO₄)₃ and aλ-MnO₂ based cathode.

FIG. 7B is a plot of Coulombic efficiency and charge/discharge capacity(as a percent of initial capacity of the cells) as a function of cyclefor a cell similar to that shown in FIG. 7A.

FIGS. 7C, 7D and 7E are plots of charge/discharge capacity as a functionof cycle for prior art cells described in S. Park et al., Journal of theElectrochemical Society, 158 (10) A1067-A1070 (2011), and Zeng et al.,Advanced Energy Materials 3 290-294, (2013).

FIGS. 8A and 8B are plots of cell voltage versus capacity (in units ofAh) and capacity versus cycle, respectively. These figures illustratethe performance of a large-format device (25 Ah) made with compositeactivated carbon/NaTi₂(PO₄)₃ anode and a λ-MnO₂ cathode.

FIG. 9 is a plot of the voltage profile (i.e., voltage versus capacityin units of Ah) of a string of four large format cells made withcomposite activated carbon/NaTi₂(PO₄)₃ anode and a λ-MnO₂ cathode undersevere over charge testing where the performance of the activated carbonmaterial is evident and labeled.

FIGS. 10A and 10B are respective plots of capacity versus cycle numberand voltage versus capacity showing the long term stability of a stringof 28 cell stacks made with composite activated carbon/NaTi₂(PO₄)₃ anodeand a λ-MnO₂ cathode

FIGS. 11A, 11B, and 11C are respective plots of voltage versus capacity,voltage versus energy and capacity versus cycle number, showing theembodied capacity (FIG. 11A), energy (FIG. 11B), and long term deepcycle life stability (FIG. 11C) of a device that contains 28prismatic/parallel cell stacks connected electrically in series with nocell level battery management.

FIGS. 12A and 12B are plots of voltage versus total capacity processedfor a string of a device having 7 series connected storage devices of 4cells each, showing evidence of the suggested self-balancing mechanismthat occurs as a result of having the activated carbon materialcomposited with the NaTi₂(PO₄)₃ material in the anode.

FIG. 13 is a cyclic voltammogram of activated carbon tested in a neutralpH aqueous solution of Na₂SO₄.

FIG. 14 is a plot of potential versus cell capacity illustratingthree-electrode data from a cell with a λ-MnO₂ cathode structure and acomposite anode containing activated carbon and NaTi₂(PO₄)₃.

DETAILED DESCRIPTION

It would be very useful to have batteries that can be built with cellsthat have a higher cell-to-cell charge storage capacity variationwithout sacrificing the integrity of the pack. The inventor hasdiscovered an aqueous electrolyte electrochemical cell that is able toself-regulate using internal electrochemical reactions upon overcharge.This self-regulation allows for high voltage strings of cells to bemanufactured with a high tolerance for cell-to-cell charge capacityvariation. Preferably, but not necessarily, the system lacks a celllevel voltage monitoring and current control circuit (also known as acell-level battery management system, or BMS). Thus, the cell levelvoltage is not monitored or controlled.

Without being bound by any particular theory, the inventor believes thatthe mechanism of self-regulation is the local electrolysis of theaqueous electrolyte that takes place at the anode electrode. Aselectrolysis occurs, a small amount of hydrogen is generated along withOH⁻ species. The OH⁻ species locally increase the pH, thereby pushingthe voltage stability window of electrolyte in the immediate vicinity ofthe anode to a lower value. This subsequently eliminates the continuedevolution of hydrogen.

It is believed that at least a portion of the hydrogen species formed oncharging of the cell is stored in, on and/or at the anode electrode ofthe cell during the period of overcharge. For brevity, the hydrogenspecies formed on charging of the cell and stored in, on and/or at theanode electrode will be referred to as “anode stored hydrogen”hereafter. It is believed that the hydrogen may be stored by beingadsorbed (e.g., by van der Waals forces) and/or chemically bound (e.g.,by covalent bonding) to the anode electrode surface and/or may be storedin the bulk of the activated carbon anode, for example by intercalationinto the activated carbon lattice, adsorption to sidewalls of theactivated carbon pores and/or by chemical bonding to the sidewalls ofactivated carbon pores. It is also possible that the hydrogen may bestored at the anode as a capacitive or pseudocapacitive double layer at(i.e., near) the anode surface. Preferably, a majority of the hydrogenspecies (e.g., at least 51%, such as 60-99%, including 70-90%) is storedin and/or at the anode electrode. Any remaining generated hydrogenspecies may evaporate from the cell as hydrogen gas.

In one preferred embodiment described in more detail below, the activematerial is a blend of a pseudocapacitive and/or capacitive material,such as activated carbon, and a high capacity, low cost sodiumintercalation material that is stable under anodic potentials in anaqueous electrolyte, such as a neutral pH aqueous electrolyte.Specifically, NaTi₂(PO₄)₃ is a non-limiting preferred intercalationmaterial which can function within a particular potential range to storealkali ions from the electrolyte during device charging. When theintercalation material is full of ions and the cell is charged further,the hydrogen interactive species then interacts with the species thatare generated during overcharge of the cell.

When the battery is allowed to discharge, it is believed that at least aportion of the anode stored hydrogen is released from the anode and isconsumed/reacted (i.e., recombines) with local OH⁻ to re-form water, orinstead diffuses to the cathode side of the cell, where it can besimilarly consumed. Preferably, a majority of the released anode storedhydrogen (e.g., at least 51%, such as 60-99%, including 70-90%) isreacted with local OH⁻ to re-form water. Any remaining released anodestored hydrogen may escape from the cell as hydrogen gas.

The inventor has discovered that the use of an anode electrode of amaterial with a high overpotential for hydrogen evolution from water,preferably a composite (e.g., blend) of the NaTi₂(PO₄)₃ intercalationmaterial and activated carbon, combined with the local electrolysis andrecombination of the aqueous electrolyte allows for an electrodeenvironment that is highly tolerant to overcharge along with having highenergy density.

An embodiment of the invention includes an electrochemical storagedevice that includes electrically connected cells (in series and/or inparallel) having a wider as-manufactured cell-to-cell variation incharge storage capacity than conventional charge storage devices. Inthis embodiment, cells with a lower charge storage capacity in the samestring of cells charge to higher potentials during cycling. When thishappens, the effect described above is believed to occur in at least oneof the cells late in the charging step with no long-term detriment tothe cell string.

In an embodiment, the electrochemical storage device is a hybridelectrochemical energy storage system in which the individualelectrochemical cells include a hybrid anode containing an alkali ionintercalation material mixed with a pseudocapacitive or double-layercapacitor material, such as activated carbon, coupled with a stableintercalation-reaction cathode material. Without wishing to be bound bya particular theory, in this system, the anode stores charge firstthrough a alkali-ion reaction with, and then through a reversiblenonfaradiac reaction of alkali (e.g., Li, Na, K, etc.) or Ca cations onthe surface of the capacitive and/or pseudocapacitive material containedin the electrode though double-layer and/or pseudocapacitance, while thecathode material undergoes a reversible faradic reaction in a transitionmetal oxide or a similar material that intercalates and deintercalatesalkali or Ca cations similar to that of a Li-ion battery.

An example of a prior art Li-based system has been described by Wang, etal., which utilizes a spinel structure LiMn₂O₄ battery electrode, anactivated carbon capacitor electrode, and an aqueous Li₂SO₄ electrolyte.Wang, et al., Electrochemistry Communications, 7:1138-42(2005). In thissystem, the negative anode electrode stores charge through a reversiblenonfaradiac reaction of Li-ion on the surface of an activated carbonelectrode. The positive cathode electrode utilizes a reversible faradiacreaction of Li-ion intercalation/deintercalation in spinel LiMn₂O₄.

A different prior art system is disclosed in U.S. patent applicationSer. No. 12/385,277, filed Apr. 3, 2009, hereby incorporated byreference in its entirety. In this system, the cathode electrodecomprises a material having a formula A_(x)M_(y)O_(z). A is one or moreof Li, Na, K, Be, Mg, and Ca, x is within a range of 0 to 1 before useand within a range of 0 to 10 during use. M comprises any one or moretransition metals, y is within a range of 1 to 3 and z is within a rangeof 2 to 7. The anode electrode comprises activated carbon and theelectrolyte comprises SO₄ ²⁻, NO₃ ⁻, ClO₄ ⁻, PO₄ ³⁻, CO₃ ²⁻, Cl⁻ or OH⁻anions. Preferably, the cathode electrode comprises a doped or undopedcubic spinel λ-MnO₂-type material or a NaMn₉O₁₈ tunnel structuredorthorhombic material, the anode electrode comprises activated carbonand the electrolyte comprises Na₂SO₄ solvated in water. The presentembodiments also differ from this prior art reference by including inthe anode material both a capacitive/pseudocapacitive material and anadditional intercalation material.

FIG. 1 is a schematic illustration of an exemplary electrochemical cell111 according to an embodiment. The cell 111 includes a cathode sidecurrent collector 130 in contact with a cathode electrode 114. Thecathode electrode 114 is in contact with an aqueous electrolyte solution121, which is also in contact with an anode electrode 112. The cell 111also includes a separator 116 located in the electrolyte solution 121 ata point between the cathode electrode 114 and the anode electrode 112.The anode electrode is also in contact with an anode side currentcollector 132. In FIG. 1, the components of the exemplary cell 111 areshown as not being in contact with each other. The cell 111 wasillustrated this way to clearly indicate the presence of the electrolytesolution 121 relative to both electrodes. However, in actualembodiments, the cathode electrode 114 is in contact with the separator116, which is in contact with the anode electrode 112.

In an embodiment, the electrochemical cell is a hybrid electrochemicalcell. That is, the cathode electrode 114 in operation reversiblyintercalates alkali metal cations and the anode electrode 112 comprisesa composite of capacitive/pseudocapacitive and intercalation basedelectrode materials which stores charge through (1) a reversibleintercalation reaction of alkali metal cations in anode electrode and(2) a capacitive/pseudocapacitive partial charge transfer surfaceinteraction with alkali metal cations on a surface of the anodeelectrode.

In one embodiment, the cell 111 is “anode limited”. That is, the chargestorage capacity of the anode electrode 112 is less than that of thecathode electrode 114. The charge storage capacity of an electrode isthe product of the mass of the electrode and the specific capacity (inunits of Ah/kg) of the electrode material. Thus, in an anode limitedcell, the mass of the active cathode material multiplied by the usablespecific capacity of the cathode material is greater than the mass ofthe active anode material multiplied by the useable specific capacity ofthe anode material. Preferably, the storage capacity of the anodeelectrode 112 available before water begins electrolysis at the anodeelectrode/electrolyte interface is 50-90%, such as 75-90% of the chargestorage capacity of the cathode electrode 114.

In a preferred embodiment, the cell is an unbalanced cell in which theproduct of the specific capacity of the anode and the load of the anodeis less than the product of the specific capacity of the cathode and theload of the cathode. For example, the cathode product may be at least20% greater, such as 50-500%, for example 100-200% greater than theanode product. Thus, the capacity (in the units of mAh) of the anode islower (such as at least 50-500% lower) than that of the cathode.

The unbalanced cell causes the water to electrolyze at high states ofcharge at only the anode (there is no oxygen generation at the cathode)and the generated hydrogen ions to become anode stored hydrogen, whenthe anode potential is below the electrolysis potential of water. Thisis not necessarily an “overcharge” condition because the battery may bedesigned to be operated at this low anode potential.

Preferably, the anode electrode 112 is made from a material that iscorrosion resistant (resistant to the hydrogen and OH species formed byelectrolysis) at the charging voltage as will be discussed below.

A method according to an embodiment includes charging the energy storagesystem 100 at a voltage 1.5 times greater and/or 0.8 volts higher than avoltage at which electrolysis of the water at the anode electrode of thecells is initiated, without inducing corrosion of the anode electrodematerial.

Cell Stack and Assembly

FIGS. 2 and 3 illustrate an electrochemical device 100, as described inU.S. application Ser. No. 13/666,452, filed on Nov. 1, 2012 (publishedas US 20013/0059185 A1) and incorporated herein by reference in itsentirety. As illustrated, the electrochemical device 100 includes ahousing 102 that includes four cavities 104. The housing 102 may havemore or fewer than four cavities 104. Each cavity is defined by walls105 of the housing 102.

Preferably, each cavity 104 includes a stack 110 of electrochemicalcells 111. Each electrochemical cell 111 includes an anode 112, acathode 114 and a separator 116 located between the anode 112 and thecathode 114. The electrochemical cells 111 may be electrically connectedin series or prismatically in the stack 110 of electrochemical cells111. In a prismatic configuration, the electrochemical cells 111 in thestack 110 are connected in parallel as illustrated in FIG. 2.

Each electrochemical cell 111 further includes two current collectors130, 132 provided to collect the current generated by theelectrochemical cells 111. The current collectors 130, 132 may be madeof any suitable electrically conducting material, such as carbon (e.g.graphite) or metal. In a prismatic stack 110, described for example inU.S. patent application Ser. No. 13/043,787 and illustrated in FIG. 2,pairs of electrochemical cells 111 are configured “front-to-front” and“back-to-back.” The cathode current collector 130 may be located inbetween cathodes 114 of adjacent electrochemical cells 111. Theresulting prismatic stack 110 therefore may include a plurality ofelectrochemical cells 111 that are stacked in pairs, front-to-front andback-to-back, alternating adjacent anode electrodes 112 and adjacentcathode electrodes 114.

Preferably, the anodes 112, cathodes 114, separators 116 and currentcollectors 130, 132 are freely stacked and are not laminated to eachother in the cavities 104. That is, no adhesives or binders are locatedbetween the individual components (anodes 112, cathodes 114, separators116 and current collectors 130, 132) in the stacks 110 as are typicallyfound in conventional laminated electrochemical cells. Instead, oneembodiment of the present invention applies a longitudinal pressureforce to a plurality of freely stacked electrochemical cells that forcesadjacent cell elements into mating contact to improve theelectrochemical reaction between the anodes and cathodes and theelectrolyte that fills the cavities as well as to improve electricalcontact between the current collectors corresponding anodes and cathodesto increase current flow to the current collectors. The anode electrode112 and/or the cathode electrode 114 may be made of two or more discretepieces, such as 4, 6, 9 or any number of discrete pieces. As illustratedin FIG. 2, the cathode electrode 114 includes 4 discrete pieces. In anembodiment, the area of the cathode current collector 130 when viewedfrom above is greater than the area of the cathode electrode pieces 114.Similarly, the area of the anode current collector 132 when viewed fromabove may be greater than the area of the anode electrode pieces 112.

In an aspect of this embodiment, the separator 116 includes flanges 116Aaround the periphery of the separator 116. The flanges 116A define oneor more cavities that are configured to receive the anode/cathodeelectrode pieces 112, 114. In another aspect, the electrochemicalstorage cells 111 include a plurality of flexible, electricallyconductive contacts (e.g., tabs) 118 operatively connected to theplurality of cathode and anode current collectors 130, 132. Theflexible, electrically conductive contacts 118 may be affixed to oneside of the cathode and anode current collectors 130, 132. In thisembodiment, electrical connection to the stacks 110 of electrochemicalstorage cells 111 in adjacent cavities 104 in the housing 102 may bemade by draping the flexible, electrically conductive contacts 118 overthe walls 105 between adjacent cavities 104 and connecting the stacks110. The stacks 110 in adjacent cavities 104 may be electricallyconnected in series, in parallel or combination thereof as desired.

FIG. 2 illustrates an embodiment of an electrochemical device 100 havingfour adjacent stacks 110 configured in a 2×2 pattern in a housing havingfour cavities 104 in the 2×2 pattern. The adjacent stacks 110 areelectrically connected in series. Alternatively, adjacent stacks may beelectrically connected in parallel.

The prismatic stack 110 also includes two electrical buses 134, 136. Oneelectrical bus 134 electrically connected to the anode currentcollectors 132 in the prismatic stack 110 and one electrical busconnected 136 to the cathode current collectors 130 in the prismaticstack 110.

In an embodiment, the electrical connection from the cathode and anodecurrent collectors 130, 132 to the electrical buses 134, 136 is via theelectrically conductive contacts 118. In this manner, theelectrochemical cells 111 in the stack 110 can be electrically connectedin parallel.

The electrochemical device 100 also includes a liquid or gel electrolyte121 (shown in FIG. 1) in the cavities 104 which substantially fills thecavity to immerse each electrochemical cell in electrolyte. The housing102 of the electrochemical device 100 is preferably hermetically sealedwith a lid 106, as shown in FIG. 3, to prevent the loss of electrolytefrom the electrochemical device 100 and a common gas volume is providedabove each cavity between the top of each cavity and the lid to allowoutgassing from all of the cavities to collect in the gas volume.

In an embodiment, the lid 106 includes a hole 160 in the center which isaligned with a corresponding hole 161 that extends through the housing102 for receiving a tie rod there through. The lid 106 may also includelid cavities 107 which are recessed to ensure that a bottom surface ofeach cavity 107 contacts the top surface of the electrode stack in thecorresponding cavity in order to transfer a pressure or compressionforce from pressure plates 202 through each unit in an assembly 200 whenthe units are stacked in an assembly, as shown in FIG. 4.

The lid cavities 107 are preferably configured to facilitate stacking ofelectrochemical devices 100 in a manner that transfer the abovedescribed pressure force from one unit device to another. The housing102 may include features to hold terminals 133 that may be connected toan outside load or to other electrochemical devices 100.

FIG. 5 illustrates another embodiment of an electrochemical energystorage system. In this embodiment, two or more of the stacks ofhousings illustrated in FIG. 4 are connected in series. In thisconfiguration, very large voltages may be conveniently generated. In analternative embodiment, two or more of the stacks of housingsillustrated in FIG. 4 are connected in parallel. In this configuration,large currents may be provided at a desired voltage.

The electrochemical devices 100 may be at elevated temperatures rangingfrom 30 to 90° C. during the charging and discharging steps to encourageion mobility both within the electrolyte and also with the electrodecrystalline structures and the porous electrode structures. One methodto perform this heating is to use controlled high current pulses throughthe devices such that they self heat.

Individual device components may be made of a variety of materials asfollows.

Composite Anode

In a preferred embodiment of the invention, the anode electrodecomprises both an ion intercalation material and a capacitive and/orpseudocapacitive material. For example, the anode electrode may comprisea mixture of a ceramic material which in operation reversiblyintercalates and deintercalates alkali metal cations from theelectrolyte and a capacitive and/or pseudocapacitive (also referred toherein as “capacitive/pseudocapacitive”) material which in operationundergoes a partial non-Faradaic charge transfer surface interactionwith alkali metal cations on a surface of the anode electrode. Thealkali or alkali earth metal cations, such as sodium, lithium,potassium, calcium, magnesium or a combination thereof aredeintercalated from the cathode into the electrolyte and thenintercalated into the anode ceramic material during the cell chargingcycle. As will be described in more detail below with respect to FIG.6A, additional alkali or alkali earth ions may be stored capacitivelyand/or pseudocapacitively in the capacitive/pseudocapacitive materialduring charging before, during and/or after the intercalation.Furthermore, hydrogen generated during the charging process may also bestored by the capacitive/pseudocapacitive material, to protect theintercalation material from being corroded by the hydrogen. The alkalior alkali earth metal cations (e.g., Na cations) are deintercalated fromthe anode into the electrolyte during cell discharge cycle (and are thenintercalated into the cathode electrode).

Any suitable ceramic intercalation materials andcapacitive/pseudocapacitive materials may be used. Preferably, thecapacitive/pseudocapacitive material comprises the activated carbondescribed above or another suitable capacitive/pseudocapacitivematerial, such as a ceramic capacitive/pseudocapacitive material or amixture thereof. Optionally, the activated carbon may be an acid washedcarbon which was subject to a nitric, sulfuric, hydrochloric, phosphoricor combinations thereof acid surface modification treatment to improveits specific capacitance and pseudocapacitive behavior, as described inU.S. published patent application US 2012/0270102 A1, which isincorporated herein by reference in its entirety. However, the acidwashing step may be omitted if desired.

Preferably, the ceramic intercalation material comprises a NASICONmaterial. As described by Vijayan et al., in chapter 4 of the“Polycrystalline Materials—Theoretical and Practical Aspects” book (Z.Zachariev, ed.), NASICON materials generally have the following formula:A_(x)B_(y)(PO₄)₃, where A is an alkali metal ion, B is a multivalentmetal ion (e.g., transition metal ion), P is at least 80 atomic percentphosphorus (e.g., 80-100 at % phosphorus and remainder (if any)transition metal(s), such as vanadium), O is oxygen and 0.95≦x≦3.05, and1.95≦y≦2.05. The charge compensating A cations occupy two types ofsites, M1 and M2 (1:3 multiplicity), in the interconnected channelsformed by corner sharing PO₄ tetrahedra and BO₆ octahedra. M1 sites aresurrounded by six oxygen atoms and located at an inversion center and M2sites are symmetrically distributed around three-fold axis of thestructure with tenfold oxygen coordination. In three-dimensionalframe-work of NASICON, numerous ionic substitutions are allowed atvarious lattice sites. Generally, NASICON structures crystallize inthermally stable rhombohedral symmetry and have a formula AB₂(PO₄)₃.Preferably, A comprises Li, Na and/or K, and B comprises Ti, Mn and/orFe. However, members of A₃M₂(PO4)₃ family (where A=Li, Na and M=Cr, Fe)crystallize in monoclinic modification of Fe₂(SO4)₃-type structure andshow reversible structural phase transitions at high temperatures.

Preferably, the anode intercalation material has a formulaAB_(2±δ1)(PO₄)_(3±δ2), where A comprises at least 5 atomic percent Na,such as 50-100 atomic percent Na, including 75-100 atomic percent Nawith the remainder (if any) being Li. Preferably, B comprises at least50 atomic percent Ti, such as 50-100 atomic percent Ti, including 75-100atomic percent Ti with the remainder (if any) being Mn or a combinationof Mn and Fe. The symbols δ1 and δ2 allow for a slight deviation fromthe strict 1:2:3 atomic ratio of alkali/transition metal/phosphate inthe material (i.e., a non-stoichiometric material is permitted). δ1 andδ2 may each independently vary between zero and 0.05, such as betweenzero and 0.01. One preferred material is NaTi₂(PO₄)₃. Alternatively, theNASICON material may comprise LiTi₂(PO₄)₃ for systems in which lithiumis used as the active ion in the electrolyte, or a mixed sodium andlithium containing NASICON material, such as Li_(1-x)Na_(x)Ti₂(PO₄)₃,where x varies from 0.05 to 0.95, such as from 0.1 to 0.9 (i.e., a solidsolution of NaTi₂(PO₄)₃ and LiTi₂(PO₄)₃), for systems in which bothsodium and lithium are used as the active ions. In general, the NASICONmaterial preferably has a formula Na_(x)Li_((1-x))Ti₂(PO₄)₃, where0.05≦x≦1.

Specifically, the present inventors have found that creating a compositeanode of NaTi₂(PO₄)₃ and surface modified activated carbon can display amarked increase in energy density (Wh/liter) compared to just theactivated carbon alone, and with more electrochemical stability thanjust the NaTi₂(PO₄)₃ alone. Without wishing to be bound by a specifictheory, it is believed that the increase in energy density and specificcapacity may be due to the increased physical density of the compositecompared to activated carbon alone. This composite has been found to becompletely stable through many cycles due to the stability of the carbonat voltage extremes, compared to the lack of stability typicallyexhibited by an electrode consisting only of NaTi₂(PO₄)₃.

The NASICON material, such as the NaTi₂(PO₄)₃ material, can be made in avariety of ways, such as a solid state method in which starting materialpowders are mixed and then heated (e.g., to decompose the initialreactants, calcine and/or sinter the material). For example, thestarting material powders may comprise sodium carbonate, anatase orrutile phase of TiO₂, and NH₄H₂PO₄ for the NaTi₂(PO₄)₃ NASICON material.The resulting NASICON material may be ground or milled into a powder andoptionally heated again (e.g., calcined and/or sintered).

The NASICON material powder is then mixed with the pseudocapacitivematerial, such as activated carbon, and optionally a binder and/or otheradditive described above (including the hydrogen storage material(s)described above), and then densified to form a composite anode. Thisresults in a composite anode which is a mixture of the NASICON andactivated carbon materials. However, in alternative embodiments, thecomposite anode may comprise discreet regions of the NASICON material inan activated carbon matrix or discreet regions of activated carbon inthe NASICON matrix, depending on the ratio of the two materials.

In one embodiment the composite anode material structure contains ablend of NaTi₂(PO₄)₃ and activated carbon (“AC”), where the blend rangesfrom 0.5:9 to 9.5:0.5 mass ratio of NaTi₂(PO₄)₃:AC, such as 1:9 to 9:1,such as 1:4 to 4:1, including 3:2 to 2:3, such as a 1:1 ratio. Theelectrode may be used in a poly-ionic aqueous electrolyte energy storagedevice (e.g., battery or hybrid device) where the anode is a freestanding electrode on a current collector and the anode contains aporous structure that is filled with electrolyte that is an aqueoussolution of an alkali-bearing salt with a pH ranging from 4 to 10. Asused herein, “poly-ionic” means usable with one or more different ions.However, the storage device may use only one ion (e.g., sodium) or acombination of ions (e.g., Na and Li) that are stored at and/or in theanode electrode. In one embodiment, the composite anode is used in an“anode limited” cell described above in which the charge storagecapacity of the anode electrode is less than that of the cathodeelectrode. However, in another embodiment, the composite anode may beused in cells which are not anode limited.

The composite anode displays a specific capacity value of at least 50mAh per gram of active material, such as 50 mAh/g to 100 mAh/g,including greater than 70 mAh/g, preferably 75 mAh/g to 100 mAh/g whencycled through a useful voltage range.

FIGS. 6A and 6B show a typical charge/discharge curve of a cell madewith a LiMn₂O₄(λ-MnO₂) cathode and a composite anode that is comprisedof a blend of activated carbon and the NaTi₂(PO₄)₃ materials. Theelectrolyte was 1 M Na₂SO₄.

FIG. 6A shows the voltage regions of different types of reactions,including capacitive/pseudocapacitive, intercalation, and over charge(including the evolution and storage of hydrogen). In the middle of thepotential versus capacity (in arbitrary units) plot, the overchargereaction dominates (e.g., at the highest voltage range and middlecapacity range), while at bottom and top states of charge (i.e., atlowest voltage range and highest and lowest capacity ranges), theactivated carbon capacitive/pseudocapacitive reaction dominates. In theintermediate voltage and capacity ranges between the overcharge and thecapacitive/pseudocapacitive regions, the anode function is dominated bythe intercalation reaction. For example, for the exemplary cell shown inFIG. 6A, the capacitive/pseudocapacitive material stores chargenon-faradiacally (i.e., capacitively and/or pseudocapacitively) at lowervoltages (e.g., 0.8 to 1.6 V), the intercalation material stores chargefaradiacally (i.e., via intercalation) at intermediate voltages (e.g.,1.6 to 1.75 V), and the hydrogen is evolved and stored by thecapacitive/pseudocapacitive material in the over charge regime athighest voltages (e.g., 1.75 to 1.9 V). The voltage ranges may differfor different cell materials and configurations.

During the charging step, the NASICON material having the formulaNa_(x)Li_((1-x))Ti₂(PO₄)₃, where 0.05≦x≦1, intercalates at least one ofLi, Na and K alkali cations from the electrolyte regardless of whichalkali species is resident in the NASICON material depending on alkalication availability in the electrolyte and intercalation affinity. Forexample, in addition to being able to intercalate Na, the NaTi₂(PO₄)₃material may also intercalate Li and/or K, even though Li and K are notresident in this material. Likewise, the Na_(x)Li_((1-x))Ti₂(PO₄)₃material which contains both lithium and sodium (i.e., where x<1) mayintercalate K in addition to or instead of Na and/or Li depending onalkali cation availability in the electrolyte and intercalationaffinity. Thus, after the charging step, one or more of Li, Na and Kintercalate and reside in the host NASICON material structure to form afully charged intercalation material that has a formulaA₂Li_(x)Na_((1-x))Ti₂(PO₄)₃, where A is one or more of Li, Na and K. Ifdesired, alkali earth ions (e.g., Mg and/or Ca) may also intercalateinto this material in addition to or instead of the alkali ions.

FIG. 6B is a plot showing the voltage vs. capacity (arbitrary units)profiles (for symmetric constant current charge/discharge studies) insingle cells containing anodes with different NaTi₂(PO₄)₃/activatedcarbon mass ratios of 20:80, 60:40 and 80:20. In general, the higher theratio of NaTi₂(PO₄)₃ to activated carbon, the more energy the cell willhave due to the higher voltage through which the energy is delivered.The capacity is provided in arbitrary units because the data is selfnormalized to show the effects and the trends of relative amounts ofactivated carbon and NaTi₂(PO₄)₃ in the anode. The result shows severalkey improvements over prior art pure activated carbon or NASICON anode.

The bulk of the energy is delivered over a more shallow voltage swingcompared to that found in devices with just a pure activated carbonanode material or an anode containing pure activated carbon and asimilar cathode. Specifically, in this case, most of the embodied energyis delivered between 1.9 and 1 V, representing a 2:1 voltage swing,which is well suited to most off the shelf large format invertersystems.

FIG. 7A is plot of voltage vs. capacity collected from a cell with ananode containing a 1:1 mass ratio of activated carbon/NaTi₂(PO₄)₃ and aλ-MnO₂ based cathode. The plot in FIG. 7A is similar to that shown inFIG. 6A, except that the capacity in FIG. 7A is plotted in units of mAh,rather than arbitrary units as in FIG. 6A.

FIG. 7B shows the cycle life stability of the test cell described abovewith respect to FIG. 7A. After more than 600 cycles (e.g., 650 cycles)there is no significant loss in function. In other words, there is nosignificant decrease in Coulombic efficiency or capacity retention as afunction of long term operation. The fluctuations in the data arebelieved to be due to thermal variation in the laboratory environment.Remarkably, even under slow cycling, there is no capacity fade observed(the capacity changed less than 5% from cycles 15-42 over more than 25cycles and less than 1% from cycles 20-42 over more than 20 cycles). Ingeneral, there was an increase in capacity between cycles 150 and 450compared to the initial capacity, and the capacity decrease was lessthan 5% over 650 cycles, such as 0% to a 10% increase in capacity. Ingeneral, the anode has a demonstrated physical and electrochemicalstability of at least 500 cycles, such as 500-650 cycles in which astate of charge swing is at least 75%, such as 75-100%, withoutdisplaying any substantial loss in energy storage function (e.g., incapacity).

This is an advance from previously published work showing theperformance of the NaTi₂(PO₄)₃ material in aqueous electrolyteenvironments, where significant capacity fade is observed. For example,as shown in FIGS. 7C-7E, the prior art cells that used a pureNaTi₂(PO₄)₃ anode and MnO₂-based cathodes in a similar electrolyte asdescribed herein were not stable over even tens of cycles. This is incontrast to the results shown in FIGS. 7A and 7B for the cells of theembodiment of the present invention, in which the capacity was stable(i.e., either increased by 1-10% or did not decrease compared to theinitial capacity) as a function of cycle number for over 600 cycles,such as about 650 cycles.

FIGS. 8A and 8B are plots of cell voltage versus capacity (in units ofAh) and capacity versus cycle, respectively. These figures illustratethe performance of a large-format device (25 Ah) made with compositeactivated carbon/NaTi₂(PO₄)₃ anode and a λ-MnO₂ cathode. The device isvery stable over many charge/discharge cycles.

If the activated carbon is not mixed with NaTi₂(PO₄)₃ material, it hasbeen found to be less stable as a functional material. Without beinglimited to a particular theory, the present inventors believe thathydrogen is evolved at extreme states of charge, such as at anovercharge condition, and that the activated carbon mixed withNaTi₂(PO₄)₃ serves several purposes during use, including protectingNaTi₂(PO₄)₃ from corrosion by gettering hydrogen species (e.g., groups)that evolve during charging, and also providing a stable material duringovercharge conditions described elsewhere herein (e.g., at a voltageabove 1.6 V). Thus, it is believed that hydrogen species (e.g., protonsor other hydrogen species) are stored pseudocapacitively at thecomposite anode electrode, while the alkali ions (e.g., Na or Na+Li) arestored by intercalation or a combination of intercalation andpseudocapacitive mechanisms in and/or at the anode electrode. Forexample, during electrochemical use, it is believed that the alkali ionintercalates and deintercalates in/out of the NaTi₂(PO₄)₃ through apotential range of −1 and −1.5 V vs. a standard mercury/mercury sulfatereference electrode. The activated carbon may also perform a chargestorage function throughout the range of use via electrochemical doublelayer capacitance (EDLC) and/or pseudocapacitance.

FIG. 9 shows the severe overcharging of a string of 4 of these devicesconnected in series, with the NaTi₂(PO₄)₃ reaction voltages, activatedcarbon (AC) voltage region, and the electrolysis/storage voltage rangealso shown. The subsequent discharge of string of cells showed higherthan usual capacity (without this excessive charge, the normal capacityis about 30 Ah). This severe over charge did not damage the function ofthe battery and it is believed that some energy was stored via thecapture and subsequent recombination of hydrogen species.

FIGS. 10A and 10B are respective plots of capacity versus cycle numberand voltage versus capacity showing the long term stability of a 28 cellstack string similar to that shown in FIG. 4 made with compositeactivated carbon/NaTi₂(PO₄)₃ anode and a λ-MnO₂ cathode. No substantialcapacity fade is observed over 145 cycles.

FIGS. 11A, 11B, and 11C are respective plots of voltage versus capacity,voltage versus energy and capacity versus cycle number. These figuresillustrates the embodied capacity (FIG. 11A), energy (FIG. 11B), andlong term deep cycle life stability (FIG. 11C) of a device that contains28 prismatic/parallel cell stack similar to that shown in FIG. 4 andconnected electrically in series with no cell level battery management.No substantial capacity fade is observed over 50 cycles.

FIGS. 12A and 12B are plots of voltage versus total capacity processedfor a string of 28 cell stacks having seven series connected storagedevices 102 of four cells stacks each, similar to that shown in FIG. 4.These figures show evidence of the suggested self-balancing mechanismthat occurs as a result of having the activated carbon materialcomposited with the NaTi₂(PO₄)₃ material in the anode. The cathodeelectrode of each cell was made from λ-MnO₂ and the anode electrode wasmade from a blend of activated carbon and NaTi₂(PO₄)₃. These cells aredesigned for 0.9 to 1.8V/cell operation. The anode electrode chargestorage capacity is 90% of the capacity of the cathode electrode.

FIG. 12B shows the behavior of the individual units 102 (i.e.,individual 4 cell stack devices) in the string, where the one of theseven units (top line) was intentionally overcharged. In this case, oneof the units was intentionally taken to an initially 20% higher state ofcharge than the others and then the entire string was charged anddischarged. The overcharged unit initially exhibits much higher voltagevalues than the other 6 units. However, with increasing total capacityprocessed, this unit converges to the highest stable voltage profile andstays there, as shown on the right of FIG. 12B, since no moreself-balancing is needed to keep the string healthy. In other words, thevoltage of the overcharged unit decreases with total capacity processeduntil is roughly approximates the voltage of the other six units in thestring.

FIG. 13 is a cyclic voltammogram that shows the increase in storagecapacitance (in Farads/g) as a result of generating local hydrogen,storing it, and then releasing it. Specifically, the figure illustratesa cyclic voltammogram of activated carbon tested in a neutral pH aqueoussolution of Na₂SO₄ in which the potential range is such that hydrogenspecies and OH⁻ species are evolved and reversibly stored locally. Bygoing to more extreme low anodic voltages (compared between lines 201and 203), more energy is stored in the material.

Specifically, line 201 is a plot of activated carbon cycled to only −1.2V vs. SME. This is a potential range where little to no hydrogen will beevolved, and the specific capacitance of plot 201 is lower than that forline 203 which shows the behavior of the carbon when it is taken to −1.6V vs. SME. In this potential range, hydrogen is evolved and the specificcapacitance of the material is increased from a maximum of about 80 F/gto a maximum of over 100 F/g (on the positive or cathodic sweep). Theadded capacitance is attributed to the storage and subsequentconsumption of hydrogen that is generated at the electrode under moreextreme potentials. In this non-limiting example, the anode activematerial is activated carbon, then electrolyte is 1 M aqueous Na₂SO₄,the sweep rate is 5 mV/second, and the reference electrode is Hg/Hg₂SO₄in sulfuric acid.

Thus, it is believed that FIG. 13 illustrates that the stored hydrogenmechanism functions in the same environment created in the hybrid devicewithin the activated carbon of the anode electrodes, such as during theabove described overcharge condition. Furthermore, the anode storedhydrogen mechanism is more pronounced for long charge/discharge cycles(e.g., >1 hour cycles, such as 2-12 hour cycle). In contrast, thismechanism may not be observed in the quick “supercapacitor” type cycles(e.g., a few seconds to a few minutes) of prior art hybrid devices, suchas the 200-920 second cycles of the Wang et al. article mentioned above.

FIG. 14 is a three electrode data set from a cell with a λ-MnO₂ cathodestructure and a composite anode containing activated carbon andNaTi₂(PO₄)₃. The figure shows the voltages of the anode and cathode withrespect to a reference electrode (and the full cell potential, which isthe cathode voltage minus the anode voltage). The data show what voltageranges in which the different electrodes work. At the highest state ofcharge just above 15 mAh, the anode potential decreases. This isbelieved to be the start of water electrolysis and hydrogen storage.

In summary, performance of the composite anode shows a specific capacitygreater than 70 mAh/g in a relevant voltage range and excellentstability during use. This is much in contrast to the performance of thepure of NaTi₂(PO₄)₃ material, which has been shown to degradesignificantly over even tens of lower rate, long duration deepdischarges in similar electrolyte environments. It is believed that thepresence of the activated carbon local to the NaTi₂(PO₄)₃ materialsabsorbs species that otherwise might contribute to the corrosion andloss of function of the material during electrochemical use.

In general, the anode may comprise any combination of materials capableof reversibly storing Na-ions (and/or other alkali or alkali earth ions)through an intercalation reaction (or phase change reaction) and surfaceadsorption/desorption (via an electrochemical double layer reactionand/or a pseudocapacitive reaction (i.e. partial charge transfer surfaceinteraction)) and be corrosion/hydrogen resistant in the desired voltagerange.

In an embodiment, the anodes are made of activated carbon (which iscorrosion free; that is, not damaged by evolved hydrogen). Thus, thecomposite negative anode electrode contains a blend of a negativeelectrode active material that can insert and extract (i.e., intercalateand deintercalate sodium and/or lithium), and a high surface area, lowelectrical conductivity activated carbon that performs the energystorage function via electrochemical double layer capacitance and/orpseudocapacitance while also having the ability to store hydrogenspecies upon overcharge without loss in function (e.g., withoutsubstantial loss or fade in capacity over at least 100 cycles, such as100-650 cycles).

Preferably, the capacitive/pseudocapacitive material of the compositeanode comprises activated carbon having a surface area of 400-3000 m²/g,such as 400-1500 m²/g, preferably 600-1500 m²/g, as determined by theBET method. Preferably the activated carbon has a high electricalresistivity, such as greater than 0.001 ohm-cm, e.g. 0.003 ohm-cm to 0.1ohm-cm. Thus, the activated carbon has an electrical resistivity that ispreferably at least two times greater than that of graphite (e.g.,0.0003 to 0.0008 ohm-cm) or other similar electrically conductive carbonmaterials which are added conductivity enhancers to prior artelectrodes.

The activated carbon is preferably modified to have a specificcapacitance more than 120 F/g (e.g., 120 to 180 F/g) in 1 M Na₂SO₄ underanodic biasing conditions. Preferably, the activated carbon ispseudocapacitive and is configured to operate in a voltage range of −1to 0.8 volts SHE. Preferably, the intercalation material within theanode has over 80 mAh/g of capacity in the voltage range of interest foranode function.

Alternative anode materials include graphite, mesoporous carbon, carbonnanotubes, disordered carbon, Ti-oxide (such as titania) materials,V-oxide materials, phospho-olivine materials, other suitable mesoporousceramic materials, other NASICON structure materials and combinationsthereof.

Optionally, the composite anode electrode includes additional materials,such as a high surface area conductive diluent (such as conducting gradegraphite, carbon blacks, such as acetylene black, non-reactive metals,and/or conductive polymers), a binder, such as PTFE, a PVC-basedcomposite (including a PVC-SiO₂ composite), cellulose-based materials,PVDF, acrylic, other non-reactive non-corroding polymer materials, or acombination thereof, plasticizer, and/or a filler.

Optionally, additional hydrogen storage material may be added to thecomposite anode material to increase the amount of anode storedhydrogen. Non-limiting examples of additional hydrogen storage materials(besides the activated carbon or other capacitive/pseudocapacitivematerial) include materials which chemically and/or physically storehydrogen, such as metal hydride materials (e.g., MgH₂, NaAlH₄, LiAlH₄,LiH, LaNi₅H₆, TiFeH₂, palladium hydride, etc.), metal hydroxidematerials, (e.g., nickel hydroxide), metal boro-hydrides (e.g., LiBH₄,NaBH₄, etc.), nanostructured carbon (e.g., carbon nanotubes, buckyballs,buckypaper, carbon nanohorns, etc.), hollow glass microspheres, etc. Thehydrogen storage material may be added only to the surface of the activeanode material, and/or it may be added to the bulk of the anode by beingmixed and pressed with the active material. The hydrogen storagematerial may be added to the anode electrode in a range of at least 0.1mass %, such as 0.5 to 10 mass %, for example 1-2 mass % of the anode.

Cathode

Any suitable material comprising a transition metal oxide, sulfide,phosphate, or fluoride can be used as active cathode materials capableof reversible alkali and/or alkali earth ion, such as Na-ionintercalation/deintercalation. Materials suitable for use as activecathode materials in embodiments of the present invention preferablycontain alkali atoms, such as sodium, lithium, or both, prior to use asactive cathode materials. It is not necessary for an active cathodematerial to contain Na and/or Li in the as-formed state (that is, priorto use in an energy storage device). However, for devices in which use aNa-based electrolyte, Na cations from the electrolyte should be able toincorporate into the active cathode material by intercalation duringoperation of the energy storage device. Thus, materials that may be usedas cathodes in embodiments of the present invention comprise materialsthat do not necessarily contain Na or other alkali in an as-formedstate, but are capable of reversible intercalation/deintercalation of Naor other alkali-ions during discharging/charging cycles of the energystorage device without a large overpotential loss.

In embodiments where the active cathode material contains alkali-atoms(preferably Na or Li) prior to use, some or all of these atoms aredeintercalated during the first cell charging cycle. Alkali cations froma sodium based electrolyte (overwhelmingly Na cations) arere-intercalated during cell discharge. This is different than nearly allof the hybrid capacitor systems that call out an intercalation electrodeopposite activated carbon. In most systems, cations from the electrolyteare adsorbed on the anode during a charging cycle. At the same time, thecounter-anions, such as hydrogen ions, in the electrolyte intercalateinto the active cathode material, thus preserving charge balance, butdepleting ionic concentration, in the electrolyte solution. Duringdischarge, cations are released from the anode and anions are releasedfrom the cathode, thus preserving charge balance, but increasing ionicconcentration, in the electrolyte solution. This is a differentoperational mode from devices in embodiments of the present invention,where hydrogen ions or other anions are preferably not intercalated intothe cathode active material. The examples below illustrate cathodecompositions suitable for Na intercalation. However, cathodes suitablefor Li, K or alkali earth intercalation may also be used.

Suitable active cathode materials may have the following general formuladuring use: A_(x)M_(y)O_(z), where A is Na or a mixture of Na and one ormore of Li, K, Be, Mg, and Ca, where x is within the range of 0 to 1,inclusive, before use and within the range of 0 to 10, inclusive, duringuse; M comprises any one or more transition metal, where y is within therange of 1 to 3, inclusive; preferably within the range of 1.5 and 2.5,inclusive; and O is oxygen, where z is within the range of 2 to 7,inclusive; preferably within the range of 3.5 to 4.5, inclusive.

In some active cathode materials with the general formulaA_(x)M_(y)O_(z), Na-ions reversibly intercalate/deintercalate during thedischarge/charge cycle of the energy storage device. Thus, the quantityx in the active cathode material formula changes while the device is inuse.

In some active cathode materials with the general formulaA_(x)M_(y)O_(z), A comprises at least 50 at % of at least one or more ofNa, K, Be, Mg, or Ca, optionally in combination with Li; M comprises anyone or more transition metal; O is oxygen; x ranges from 3.5 to 4.5before use and from 1 to 10 during use; y ranges from 8.5 to 9.5 and zranges from 17.5 to 18.5. In these embodiments, A preferably comprisesat least 51 at % Na, such as at least 75 at % Na, and 0 to 49 at %, suchas 0 to 25 at %, Li, K, Be, Mg, or Ca; M comprises one or more of Mn,Ti, Fe, Co, Ni, Cu, V, or Sc; x is about 4 before use and ranges from 0to 10 during use; y is about 9; and z is about 18.

In some active cathode materials with the general formulaA_(x)M_(y)O_(z), A comprises Na or a mix of at least 80 atomic percentNa and one or more of Li, K, Be, Mg, and Ca. In these embodiments, x ispreferably about 1 before use and ranges from 0 to about 1.5 during use.In some preferred active cathode materials, M comprises one or more ofMn, Ti, Fe, Co, Ni, Cu, and V, and may be doped (less than 20 at %, suchas 0.1 to 10 at %; for example, 3 to 6 at %) with one or more of Al, Mg,Ga, In, Cu, Zn, and Ni.

General classes of suitable active cathode materials include (but arenot limited to) the layered/orthorhombic NaMO₂ (birnessite), the cubicspinel based manganate (e.g., MO₂, such as λ-MnO₂ based material where Mis Mn, e.g., Li_(x)M₂O₄ (where 1≦x<1.1) before use and Na₂Mn₂O₄ in use),the Na₂M₃O₇ system, the NaMPO₄ system, the NaM₂(PO₄)₃ system, theNa₂MPO₄F system, the tunnel-structured orthorhombic NaM₉O₁₈, ormaterials with the Prussian blue type crystal structure having a formulaKMFe(CN)₆, where M in all formulas comprises at least one transitionmetal. Typical transition metals may be Mn or Fe (for cost andenvironmental reasons), although Co, Ni, Cr, V, Ti, Cu, Zr, Nb, W, Zn,Mo (among others), or combinations thereof, may be used to wholly orpartially replace Mn, Fe, or a combination thereof. In embodiments ofthe present invention, Mn is a preferred transition metal.

However, in other embodiments, the material may lack Mn. For example,for materials having a Prussian blue type crystal structure, such as thePrussian blue hexacyanometallate crystal structure, M may be copper andthe material may comprise copper hexacyanoferrate, KMFe(CN)₆. Othermetal-hexacyanoferrate materials may also be used, where the M is one ormore of some combination of Cu, Ni, Fe, Ti, Mn, or other transitionmetals, such as Zn and/or Co. Examples of these materials are describedin C. Wessells et al., Nature Communications 2, article number 550,published Nov. 22, 2011 (doi:10.1038/ncomms1563) and Y. Lu et al., Chem.Commun., 2012, 48, 6544-6546, both of which are incorporated herein byreference in their entirety.

In some embodiments, cathode electrodes may comprise multiple activecathode materials, either in a homogenous or near homogenous mixture orlayered within the cathode electrode.

In some embodiments, the initial active cathode material comprisesNaMnO₂ (birnassite structure) optionally doped with one or more metals,such as Li or Al.

In some embodiments, the initial active cathode material comprisesλ-MnO₂ (i.e., the cubic isomorph of manganese oxide) based material,optionally doped with one or more metals, such as Li or Al.

In these embodiments, cubic spinel λ-MnO₂ may be formed by first forminga lithium containing manganese oxide, such as lithium manganate (e.g.,cubic spinel LiMn₂O₄) or non-stoichiometric variants thereof. Inembodiments which utilize a cubic spinel λ-MnO₂ active cathode material,most or all of the Li may be extracted electrochemically or chemicallyfrom the cubic spinel LiMn₂O₄ to form cubic spinel λ-MnO₂ type material(i.e., material which has a 1:2 Mn to O ratio, and/or in which the Mnmay be substituted by another metal, and/or which also contains analkali metal, and/or in which the Mn to O ratio is not exactly 1:2).This extraction may take place as part of the initial device chargingcycle. In such instances, Li-ions are deintercalated from the as-formedcubic spinel LiMn₂O₄ during the first charging cycle. Upon discharge,Na-ions from the electrolyte intercalate into the cubic spinel λ-MnO₂.As such, the formula for the active cathode material during operation isNa_(y)Li_(x)Mn₂O₄ (optionally doped with one or more additional metal asdescribed above, preferably Al), with 0<x<1, 0<y<1, and x+y≦1.1.Preferably, the quantity x+y changes through the charge/discharge cyclefrom about 0 (fully charged) to about 1 (fully discharged). However,values above 1 during full discharge may be used. Furthermore, any othersuitable formation method may be used. Non-stoichiometric Li_(x)Mn₂O₄materials with more than 1 Li for every 2 Mn and 4 O atoms may be usedas initial materials from which cubic spinel λ-MnO₂ may be formed (where1≦x<1.1 for example). Thus, the cubic spinel λ-manganate may have aformula Al_(z)Li_(x)Mn_(2-z)O₄ where 1≦x<1.1 and 0≦z<0.1 before use, andAl_(z)Li_(x)Na_(y)Mn₂O₄ where 0≦x<1.1, 0≦x<1, 0≦x+y<1.1, and 0≦z<0.1 inuse (and where Al may be substituted by another dopant).

In some embodiments, the initial cathode material comprises Na₂Mn₃O₇,optionally doped with one or more metals, such as Li or Al.

In some embodiments, the initial cathode material comprises Na₂FePO₄F,optionally doped with one or more metals, such as Li or Al.

In some embodiments, the cathode material comprises orthorhombicNaM₉O₁₈, optionally doped with one or more metals, such as Li or Al.This active cathode material may be made by thoroughly mixing Na₂CO₃ andMn₂O₃ to proper molar ratios and firing, for example at about 800° C.The degree of Na content incorporated into this material during firingdetermines the oxidation state of the Mn and how it bonds with O₂locally. This material has been demonstrated to cycle between0.33<x<0.66 for Na_(x)MnO₂ in a non-aqueous electrolyte.

In another embodiment, the cathode material comprises cubic spinelLiMn₂O₄ and the electrolyte comprises Li₂SO₄, a blend of Li₂SO₄ andNa₂SO₄, or Na₂SO₄ only, and the anode comprises a composite of activatedcarbon and NaTi₂(PO₄)₃. In this case, a true mixed ion system ispossible where either the anode and/or the cathode mayintercalate/deintercalate both Li and/or Na ions during the normalcourse of use. This particular embodiment is thought to be aparticularly low cost solution on a price/energy basis when consideringthe cost of the materials in the electrodes and the energy they embody.

Optionally, the cathode electrode may be in the form of a compositecathode comprising one or more active cathode materials, a high surfacearea conductive diluent (such as conducting grade graphite, carbonblacks, such as acetylene black, non-reactive metals, and/or conductivepolymers), a binder, a plasticizer, and/or a filler. Exemplary bindersmay comprise polytetrafluoroethylene (PTFE), a polyvinylchloride(PVC)-based composite (including a PVC-SiO₂ composite), cellulose-basedmaterials, polyvinylidene fluoride (PVDF), hydrated birnassite (when theactive cathode material comprises another material), other non-reactivenon-corroding polymer materials, or a combination thereof. A compositecathode may be formed by mixing a portion of one or more preferredactive cathode materials with a conductive diluent, and/or a polymericbinder, and pressing the mixture into a pellet. In some embodiments, acomposite cathode electrode may be formed from a mixture of about 50 to90 wt % active cathode material, with the remainder of the mixturecomprising a combination of one or more of diluent, binder, plasticizer,and/or filler. For example, in some embodiments, a composite cathodeelectrode may be formed from about 80 wt % active cathode material,about 10 to 15 wt % diluent, such as carbon black, and about 5 to 10 wt% binder, such as PTFE.

One or more additional functional materials may optionally be added to acomposite cathode to increase capacity and replace the polymeric binder.These optional materials include but are not limited to Zn, Pb, hydratedNaMnO₂ (birnassite), and Na₄Mn₉O₁₈ (orthorhombic tunnel structure). Ininstances where hydrated NaMnO₂ (birnassite) and/or hydratedNa_(0.44)MnO₂ (orthorhombic tunnel structure) is added to a compositecathode, the resulting device has a dual functional material compositecathode. A cathode electrode will generally have a thickness in therange of about 40 to 800 μm.

Current Collectors

In embodiments of the present invention, the cathode and anode materialsmay be mounted on current collectors. For optimal performance, currentcollectors are desirable that are electronically conductive andcorrosion resistant in the electrolyte (aqueous Na-cation containingsolutions, described below) at operational potentials.

For example, an anode current collector should be stable in a range ofapproximately −1.2 to −0.5 V vs. a standard Hg/Hg₂SO₄ referenceelectrode, since this is the nominal potential range that the anode halfof the electrochemical cell is exposed during use. A cathode currentcollector should be stable in a range of approximately 0.1 to 0.7 V vs.a standard Hg/Hg₂SO₄ reference electrode.

Suitable uncoated current collector materials for the anode side includestainless steel, Ni, NiCr alloys, Al, Ti, Cu, Pb and Pb alloys,refractory metals, and noble metals. Alternatively, electricallyconductive carbon, such as graphite, may be used.

Suitable uncoated current collector materials for the cathode sideinclude stainless steel, Ni, NiCr alloys, Ti, Pb-oxides (PbO_(x)), andnoble metals. Alternatively, electrically conductive carbon, such asgraphite, may be used.

Current collectors may comprise solid foils, sheet or mesh materials.For example, graphite sheet current collectors 130 and 132 may be used,as shown in FIG. 2.

Another approach is to coat a metal foil current collector of a suitablemetal, such as Al, with a thin passivation layer that will not corrodeand will protect the foil onto which it is deposited. Such corrosionresistant layers may be, but are not limited to, TiN, CrN, C, CN, NiZr,NiCr, Mo, Ti, Ta, Pt, Pd, Zr, W, FeN, CoN, etc. These coated currentcollectors may be used for the anode and/or cathode sides of a cell. Inone embodiment, the cathode current collector comprises Al foil coatedwith TiN, FeN, C, or CN. The coating may be accomplished by any methodknown in the art, such as but not limited to physical vapor depositionsuch as sputtering, chemical vapor deposition, electrodeposition, spraydeposition, or lamination.

Electrolyte

Embodiments of the present invention provide a secondary (rechargeable)energy storage system which uses a water-based (aqueous) electrolyte,such as an alkali based (e.g., Li and/or Na-based) or alkaline earthbased aqueous electrolyte. Use of Na allows for use of much thickerelectrodes, much less expensive separator and current collectormaterials, and benign and more environmentally friendly materials forelectrodes and electrolyte salts. Additionally, energy storage systemsof embodiments of the present invention can be assembled in an open-airenvironment, resulting in a significantly lower cost of production.

Electrolytes useful in embodiments of the present invention comprise asalt dissolved fully in water. For example, the electrolyte may comprisea 0.1 M to 10 M solution of at least one anion selected from the groupconsisting of SO₄ ²⁻, NO₃ ⁻, ClO₄ ⁻, PO₄ ³⁻, CO₃ ²⁻, CH₃COO⁻, Cl⁻,and/or OH⁻. Thus, Na cation containing salts may include (but are notlimited to) Na₂SO₄, NaNO₃, NaClO₄, Na₃PO₄, Na₂CO₃, NaCl, and NaOH, or acombination thereof.

In some embodiments, the electrolyte solution may be substantially freeof Na. In these instances, cations in salts of the above listed anionsmay be an alkali other than Na (such as Li or K) or alkaline earth (suchas Ca, or Mg) cation. Thus, alkali other than Na cation containing saltsmay include (but are not limited to) Li₂SO₄, LiNO₃, LiClO₄, Li₃PO₄,Li₂CO₃, LiCl, and LiOH, K₂SO₄, KNO₃, KClO₄, K₃PO₄, K₂CO₃, KCl, and KOH.Exemplary alkaline earth cation containing salts may include CaSO₄,Ca(NO₃)₂, Ca(ClO₄)₂, CaCO₃, and Ca(OH)₂, MgSO₄, Mg(NO₃)₂, Mg(ClO₄)₂,MgCO₃, and Mg(OH)₂. Electrolyte solutions substantially free of Na maybe made from any combination of such salts. In this embodiment, thecathode electrode preferably comprises a doped or undoped cubic spinelLiMn₂O₄, the electrolyte preferably comprises at least one of Li₂SO₄,LiClO₄, LiNO₃, or MnClO₄ solvated in water, and the electrolyte issodium free and contains no solvated sodium ions, but contains one ormore of Li, K, Ca, Mn and Mg solvated cations.

In other embodiments, the electrolyte solution may comprise a solutionof a Na cation containing salt and one or more non-Na cation containingsalt. For example, as noted above, the electrolyte may comprise bothsodium and lithium containing salts (e.g., lithium sulfate and sodiumsulfate) solvated in water. In this embodiment, the cathode may comprisethe cubic spinel λ-MnO₂ or LiMn₂O₄, and the anode may comprise a mixtureof activated carbon and a mixed sodium and lithium containing NASICONmaterial, such as Li_(1-x)Na_(x)Ti₂(PO₄)₃, where x varies from 0.05 to0.95.

In one embodiment, it may also be advantageous to saturate theelectrolyte with metallic species such that they may not be leached outof the active materials. For example, dissolving an excess of Mn ions inthe electrolyte can combat the subsequent dissolution of Mn from theelectrodes if they were to contain Mn. For example, the electrolyte maycomprise Na₂SO₄ solvated in water and saturated with a MnClO₄ salt suchthat no Mn is able to be dissolved into the electrolyte from the Mncontaining cathode (e.g., such as the spinel manganese oxide cathode)during the charging and discharging steps. Alternatively, theelectrolyte may be completely saturated with one or more of Na₂SO₄,Li₂SO₄, NaClO₄, LiClO₄, NaNO₃, or LiNO₃ salts solvated in water suchthat no manganese ions dissolve into the electrolyte from the cathodeduring the steps of charging and discharging.

Molar concentrations preferably range from about 0.05 M to 3 M, such asabout 0.1 to 1 M, at 100° C. for Na₂SO₄ in water depending on thedesired performance characteristics of the energy storage device, andthe degradation/performance limiting mechanisms associated with highersalt concentrations. Similar ranges are preferred for other salts.

A blend of different salts (such as a blend of a sodium containing saltwith one or more of an alkali, alkaline earth, lanthanide, aluminum andzinc salt) may result in an optimized system. Such a blend may providean electrolyte with sodium cations and one or more cations selected fromthe group consisting of alkali (such as Li or K), alkaline earth (suchas Mg and Ca), lanthanide, aluminum, and zinc cations.

The pH of the electrolyte may be neutral (e.g., close to 7 at roomtemperature, such as 6.5 to 7.5). Optionally, the pH of the electrolytemay be altered by adding some additional OH— ionic species to make theelectrolyte solution more basic, for example by adding NaOH other OH⁻containing salts, or by adding some other OH⁻ concentration-affectingcompound (such as H₂SO₄ to make the electrolyte solution more acidic).The pH of the electrolyte affects the range of voltage stability window(relative to a reference electrode) of the cell and also can have aneffect on the stability and degradation of the active cathode materialand may inhibit proton (H⁺) intercalation, which may play a role inactive cathode material capacity loss and cell degradation. In somecases, the pH can be increased to 11 to 13, thereby allowing differentactive cathode materials to be stable (than were stable at neutral pH7). In some embodiments, the pH may be within the range of about 3 to13, such as between about 3 and 6 or between about 8 and 13.

Optionally, the electrolyte solution contains an additive for mitigatingdegradation of the active cathode material, such as birnassite material.An exemplary additive may be, but is not limited to, Na₂HPO₄, inquantities sufficient to establish a concentration ranging from 0.1 mMto 100 mM.

Separator

A separator for use in embodiments of the present invention may comprisea woven or non-woven cotton sheet, PVC (polyvinyl chloride), PE(polyethylene), glass fiber or any other suitable material.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the invention is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the invention. All of thepublications, patent applications and patents cited herein areincorporated herein by reference in their entirety.

What is claimed is:
 1. An anode electrode suitable for an aqueouselectrolyte energy storage device, the anode electrode comprising asolid mixture of a NASICON material and an electrochemical double layercapacitive and/or pseudocapacitive material comprising an activatedcarbon having an electrical resistivity greater than 0.001 ohm-cm. 2.The electrode of claim 1, wherein the NASICON material formula isA_(x)B_(y)(PO₄)₃, where A is an alkali ion, or combination of alkaliions, B is a multivalent metal ion, P is at least 80 atomic percentphosphorus, O is oxygen, 0.95≦x≦3.05, and 1.95≦y≦2.05.
 3. The electrodeof claim 2, wherein the NASICON material formula is AB_(2±δ1)(PO₄)_(3±δ2), where A comprises at least 5 atomic percent Na, Bcomprises at least 50 atomic percent Ti, and δ1 and δ2 eachindependently vary between zero and 0.05.
 4. The electrode of claim 3,wherein the NASICON material is a solid solution ofNa_(x)Li_((1-x))Ti₂(PO₄)₃, where 0.05≦x≦1 and the activated carbon has asurface area between 400 and 1500 m²/g.
 5. The electrode of claim 4,wherein the anode electrode comprises a mixture of a NaTi₂(PO₄)₃ NASICONmaterial and the activated carbon, the mixture having aNaTi₂(PO₄)₃:activated carbon mass ratio ranging from 0.5:9 to 9.5:0.5.6. The electrode of claim 1, wherein the anode electrode has a specificcapacity of at least 50 mAh/g in aqueous electrolyte with alkali ions asfunctional ions.
 7. The electrode of claim 1 wherein the anode electrodehas a physical and electrochemical stability of at least 500 cycles inwhich a state of charge swing is at least 75% without displaying anyloss in energy storage function.
 8. An energy storage device,comprising: a plurality of electrochemical energy storage cellsconnected electrically in parallel or series, wherein each cellcomprises: a negative anode electrode of claim 1; a positive cathodeelectrode; a separator; and an aqueous electrolyte, wherein the chargestorage capacity of the anode electrode is less than the charge storagecapacity of the cathode; wherein water in the electrolyte locallyelectrolyzes to form hydrogen and OH⁻ species at the anode electrode ofat least one of the plurality of cells when the charge storage capacityof the ion insertion material resident in the anode electrode isexceeded upon charging of the device; and wherein alkali cation speciesfrom the electrolyte insert and extract into and out of the NASICONmaterial regardless of what kind of alkali species is resident in theNASICON material.
 9. The device of claim 8, wherein: the devicecomprises a hybrid energy storage device; the cathode electrode inoperation reversibly inserts and extracts alkali metal cations; and theanode electrode stores the hydrogen species capacitively orpseudocapacitively and stores the metal cations through a combination ofion insertion and at least one of capacitive and pseudocapacitivestorage.
 10. The device of claim 9, wherein the activated carbon has asurface area of 600-3000 m²/g as determined by the BET method, theinsertion material comprises NaTi₂(PO₄)₃, and in use, the activatedcarbon protects the NaTi₂(PO₄)₃ material from corrosion by getteringcorrosive species comprising at least one of OH⁻ and hydrogen speciesthat evolve during charging.
 11. The device of claim 8, wherein thedevice comprises a plurality of cells grouped and connected in parallel,where the groups of parallel connected cells are then connectedelectrically in series with no cell-level battery management system. 12.The device of claim 8, wherein: the storage capacity of the anodeelectrode available before water begins electrolysis at the anodeelectrode/electrolyte interface is 50-90% of the charge storage capacityof the cathode electrode; a mass ratio of the anode electrode to thecathode electrode is less than 1; and an ion storage capacity of theanode in mAh is less than an ion storage capacity of the cathode in mAh.13. The device of claim 9, wherein: the cathode electrode comprises amaterial having a formula A_(x)M_(y)O_(z), where A is one or more of Li,Na, K, Be, Mg, and Ca, where x is within a range of 0 to 1, before useand within a range of 0 to 10, during use; M comprises any one or moretransition metals, where y is within a range of 1 to 3, and z is withina range of 2 to 7; the anode electrode comprises a combination ofactivated carbon and NaTi₂(PO₄)₃; and the electrolyte comprises one ormore of 0.1 to 10 M SO₄ ²⁻, NO₃ ⁻, ClO₄ ⁻, PO₄ ³⁻, CO₃ ²⁻, Cl⁻, CH₃COO⁻or OH⁻ anions, one or more of 0.1 to 10 M Li⁺, Na⁺, K⁺, Ca²⁺ or Mg²⁺cations, and a pH of 4-10.
 14. The device of claim 13, wherein thecathode electrode comprises a doped or undoped cubic spinel λ-MnO₂-typematerial or a Na_(0.44)MnO₂ tunnel structured orthorhombic material, andthe electrolyte comprises at least one of Na₂SO₄, Li₂SO₄, NaClO₄,LiClO₄, NaNO₃, LiNO₃, or MnClO₄ solvated in water.
 15. The device ofclaim 13, wherein: the cathode electrode comprises a doped or undopedcubic spinel LiMn₂O₄; the electrolyte comprises at least one of Li₂SO₄,LiClO₄, LiNO₃, or MnClO₄ solvated in water; and the electrolyte issodium free and contains no solvated sodium ions, but contains one ormore of Li, K, Ca, Mn, and Mg solvated cations.
 16. The device of claim9, wherein the cathode electrode comprises a material having a formulaKMFe(CN)₆, wherein M is at least one transition metal, and a Prussianblue type crystal structure, and the electrolyte comprises at least oneof Na₂SO₄, K₂SO₄ or Li₂SO₄ solvated in water.
 17. The device of claim 8,wherein: the OH⁻ species increase a pH proximal to the anode electrodesurface and wherein the increase in pH lowers the voltage stabilitywindow of the electrolyte locally, thereby reducing or eliminatingfurther hydrogen evolution; and the hydrogen species formed on chargingof the at least one cell combines with the OH′ species upon dischargingof the same at least one cell.
 18. The device of claim 17, wherein atleast a portion of the hydrogen species formed on charging of the atleast one cell is stored in, on or at the anode electrode.
 19. Thedevice of claim 18, wherein the anode electrode further comprises ahydrogen storage material.
 20. The device of claim 8, wherein: a firstcell of the plurality of cells in the device comprises a lower chargestorage capacity as manufactured than a second cell of the plurality ofcells in the device; the first cell experiences overcharge andundercharge conditions during discharging and charging; and the devicelacks a cell level voltage monitoring and a current control circuit. 21.The electrode of claim 1, wherein the anode electrode comprises a solidfree standing anode electrode on a current collector and the anodeelectrode comprises a porous crystalline structure that is filled withthe electrolyte.
 22. The electrode of claim 1, wherein the anodeelectrode comprises a solid solution of the ion intercalation materialand the electrochemical double layer capacitive and/or pseudocapacitivematerial having a porous crystalline structure that is filled with theelectrolyte.